Cognitive flow control based on channel quality conditions

ABSTRACT

A system and method which improve the performance of a wireless transmission system by intelligent use of the control of the flow of data between a radio network controller (RNC) and a Node B. The system monitors certain criteria and, if necessary, adaptively increases or decreases the data flow between the RNC and the Node B. This improves the performance of the transmission system by allowing retransmitted data, signaling procedures and other data to be successfully received at a faster rate, by minimizing the amount of data buffered in the Node B. Flow control is exerted to reduce buffering in the Node B upon degradation of channel qualities, and prior to a High Speed Downlink Shared Channel (HS-DSCH) handover.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/444,283, filed Apr. 11, 2012, which is a continuation of U.S. patentapplication Ser. No. 10/431,897, filed May 8, 2003, which issued as U.S.Pat. No. 8,175,030 on May 8, 2012, which claims priority from U.S.Patent Application No. 60/379,858, filed May 10, 2002, which areincorporated by reference as if fully set forth.

FIELD OF INVENTION

The present invention relates to the field of wireless communications.More specifically, the present invention relates to the exertion of flowcontrol for data transmissions between a radio network controller (RNC)and a Node B in a third generation (3G) telecommunication system.

BACKGROUND

A 3G Universal Terrestrial Radio Access Network (UTRAN) comprisesseveral RNCs, each of which is associated with one or more Node Bs, andeach Node B being further associated with one or more cells.

The 3G Frequency Division Duplex (FDD) and Time Division Duplex (TDD)modes typically use the RNC to distribute (i.e., buffer and schedule),data transmissions to at least one User Equipment (UE). However, for thehigh speed channels of a 3G cellular system, data is scheduled fortransmission by the Node B. One of these high speed channels, forexample, is the High Speed Downlink Shared Channel (HS-DSCH). Since datais scheduled by the Node B, it is necessary to buffer data in the Node Bfor transmission to the UE(s).

There are many scenarios where large amounts of data buffered in theNode B have a negative impact on the overall operation of the system.Several of these scenarios will be described hereinafter.

The first scenario is related to the retransmission mechanisms in 3Gsystems to achieve high reliability of end-to-end data transmissions. Itwould be understood by those of skill in the art that the transmissionfailure between the Node B and the UE could be due to many differentreasons. For example, the Node B may have retried the transmissionseveral times without success. Alternatively, the transmission timeallotted for a particular transmission may have expired. The presentinvention which will be described in further detail hereinafter isintended to cover both these situations and any other situations wherethe failure of a data transmission necessitates a radio link control(RLC) retransmission.

There are many levels of retransmission mechanisms. One mechanism, forexample, is the retransmissions of the Hybrid Automatic Repeat Request(H-ARQ) process for High Speed Downlink Packet Access (HSDPA). The H-ARQprocess provides a mechanism where transmissions that are received inerror are indicated to the transmitter, and the transmitter retransmitsthe data until the data is received correctly.

In addition to the H-ARQ process, there are entities in the RNC and theUE. The sending RLC entity signals a sequence number (SN) in the headerof a particular protocol data unit (PDU) which is used by the receivingRLC entity to ensure that no PDUs are missed in the transmission. Ifthere are PDUs missed during the transmission, as realized by anout-of-sequence delivery of PDUs, the receiving RLC entity sends astatus report PDU to inform the sending RLC entity that certain PDUs aremissing. The status report PDU describes the status of successful and/orunsuccessful data transmissions. It identifies the SNs of the PDUs thatare missed or received. If a PDU is missed, the sending RLC entity willretransmit a duplicate of the missed PDU to the receiving RLC entity.

The impact of retransmissions in system performance will be describedwith reference to FIG. 1. As shown, when the PDU with SN=3 is notreceived successfully by the UE, the RLC within the UE requests its peerentity in the RNC for a retransmission. In the interim, the PDUs withSNs=6 and 7 are queued in the buffer of the Node B.

Referring to FIG. 2, since the retransmission process takes a finiteamount of time and data is continuing to be transmitted, two more PDUswith SNs=8 and 9 have queued up behind the PDUs with SNs=6 and 7, and infront of the retransmitted PDU with SN=3. The PDU with SN=3 will have towait until the PDUs with SNs=6-9 have been transmitted to the UE.Additionally, due to the requirement of in-sequence delivery of data tohigher layers, the PDUs with SNs=4-9 will not be passed through higherlayers until the PDU with SN=3 is received and in-sequence delivery ofdata can be performed.

The UE will be required to buffer the out-of-sequence data until themissing PDU can be transmitted. This not only results in a delay of thetransmission, but requires the UE to have a memory capable of databuffering for continuous data reception until the missed data can besuccessfully retransmitted. Otherwise, the effective data transmissionrate is reduced, thereby affecting quality of service. Since memory isvery expensive, this is an undesirable design constraint. Accordingly,this first scenario is when there is a need for RLC retransmission and alarge amount of data buffered in the Node B results in a larger dataretransmission delay and higher UE memory requirements.

A second scenario when the buffering of data in the Node B negativelyaffects system performance is in the case that layer 2 (L2) or layer 3(L3) messages and data transmissions are processed by the samescheduling processes or share a single buffer in the Node B. While datais being buffered and processed and an L2/L3 message comes behind it,the message cannot circumvent the transmission queue. The greater theamount of data within a transmission buffer, (which operates asfirst-in-first-out (FIFO) buffer), the longer it takes for an L2/L3message or data to get through the buffer. Any higher priority L2/L3messages are thus delayed by the data in the buffers.

A third scenario where the buffering of data in the Node B couldnegatively impact the performance of the system is in the event of aserving HS-DSCH cell change. Since the Node B performs scheduling andbuffering of data for an HS-DSCH, when the UE performs a serving HS-DSCHcell change from a source Node B to a target Node B, there is apossibility that considerable amounts of data may still be buffered inthe source Node B after the handover. This data is not recoverablebecause there is no mechanism that exists within the UTRAN architecturefor data buffered as the source Node B to be transmitted to the targetNode B. Upon a serving HS-DSCH cell change, the RNC has no informationregarding how much, if any, data is lost since the RNC it does not knowwhat data is buffered in the source Node B. The greater the amount ofdata that is buffered in the Node B in the event of an HS-DSCH cellchange, the greater the amount of data which will ultimately be strandedin the source Node B and will have to be retransmitted.

Accordingly, it would be desirable for the aforementioned reasons tolimit the amount of data that is buffered in the Node B.

SUMMARY

The present invention is a system and method which improve theperformance of a wireless transmission system by intelligent use of thecontrol of the flow of data between the RNC and the Node B. The systemmonitors certain criteria and, if necessary, adaptively increases ordecreases the data flow between the RNC and the Node B. This improvesthe performance of the transmission system by allowing retransmitteddata, signaling procedures and other data to be successfully received ata faster rate than in prior art systems, by minimizing the amount ofdata buffered in the Node B. Flow control is exerted to reduce bufferingin the Node B upon degradation of the channel quality, and prior to anHS-DSCH handover.

In a preferred embodiment, the present invention is implemented in awireless communication system including a radio network controller (RNC)in communication with a Node B having at least one buffer therein forstoring data. The RNC signals the Node B with a request that the RNCsend a certain amount of data to the Node B. The Node B monitors aselected quality indicator and calculates a capacity allocation for thebuffer based on the selected quality indicator. The Node B signals thecapacity allocation to the RNC. In response to receipt of the capacityallocation, the RNC transmits data to the Node B at a data flow ratedetermined in accordance with the capacity allocation and at least onepredetermined criterion. The Node B adjusts the buffer accordingly.

BRIEF DESCRIPTION OF THE DRAWING(S)

A more detailed understanding of the invention may be had from thefollowing description, given by way of example and to be understood inconjunction with the accompanying drawings wherein:

FIG. 1 shows prior art buffering of data in the RNC, the Node B and theUE.

FIG. 2 shows prior art buffering of data in the RNC, the Node B and theUE in the event of a retransmission.

FIGS. 3A and 3B, taken together, are a method in accordance with thepresent invention for monitoring the channel quality and adjusting theflow of data between the RNC and the Node B.

FIG. 4 is the buffering of data in the RNC, the Node B and the UE in theevent of a retransmission using the method of FIGS. 3A and 3B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention will be described with reference to the drawingfigures wherein like numeral represent like elements throughout.Although the present invention will be described by referring to aspecific number of PDUs being queued in a buffer (such as ten PDUs),this number of PDUs is referred to only for simplicity. The actualnumber of PDUs which is being transmitted and buffered in accordancewith the aforementioned scenarios is more likely on the order of severalhundred PDUs or more. The present invention and the teachings herein areintended to be applicable to any number of PDUs and any sizetransmission buffer.

In general, the present invention reduces the flow of data to the Node Bfor a UE when there is a degradation of channel quality of the UE, andincreases the flow of data to the Node B when there is an improvement inchannel quality of the UE. In order to control the flow of thetransmission of data between the RNC and the Node B, the presentinvention monitors one or more parameters for channel quality. This flowcontrol can be based on one criterion, or a combination of manydifferent criteria. Additionally, as will be explained in detailhereinafter, the criterion may be internally-generated by the Node B, ormay be generated by an external entity, (such as the UE), and sent tothe Node B.

Referring to FIG. 3A, a method 50 in accordance with the presentinvention for monitoring the quality of a communication channel andadjusting the flow of data between the RNC 52 and the Node B 54 isshown. This method 50 handles the transmission of data between the RNC52 and the Node B 54. The RNC 52 transmits a capacity request to theNode B 54 (step 58). The capacity request is basically a request fromthe RNC 52 to the Node B 54 that the RNC 52 would like to send a certainamount of data to the Node B 54. The Node B 54 receives the capacityrequest and monitors the selected quality indicator (step 60). Thisselected quality indicator may be based upon data transmitted from theUE (as will be described in detail hereinafter), or may be based upon aninternally-generated quality indicator, such as the depth of the bufferin the Node B 54.

The Node B 54 also monitors the status of the buffer within the Node B(step 62). As would be understood by those with skill in the art,although the present invention is described with reference to a singlebuffer within the Node B 54 for simplicity, most likely the buffercomprises a plurality of buffers or a single buffer segmented into aplurality of sub-buffers, each buffer or sub-buffer being associatedwith one or more data flows. Regardless of whether there is one or moremultiple buffers, an indicator which indicates the amount of data withinthe buffer is generated internally within the Node B. This permits theNode B 54 to monitor the amount of data within the buffer, and also tomonitor the amount of additional data the buffer may accept.

The Node B 54 calculates and transmits a capacity allocation (step 64)to the RNC 52. The capacity allocation is an authorization by the Node B54 to permit the RNC 52 to transmit a certain amount of data. The RNC52, upon receiving the capacity allocation, transmits the data inaccordance with the allocation (step 66). That is, the RNC 52 sends datato the Node B 54, the amount of which may not exceed the capacityallocation. The Node B then adjusts its buffer accordingly to receiveand store the data (step 69). The amount of data stored in the bufferwill change in accordance with the incoming data that is transmittedfrom the RNC 52 and the outgoing data that is transmitted to the UE 82(shown in FIG. 3 b).

It would be appreciated by those of skill in the art that the method 50shown in FIG. 3A is constantly repeated as data flows from the RNC 52 tothe Node B 54, and as the flow rate is continually adjusted by the NodeB 54. It should also be noted that method steps 58, 60, 62, 64, 66 and69 are not necessarily performed in sequence, and any one step may beapplied multiple times before a different step in method 50 is applied.Additionally, some of the steps, such as the capacity allocation step64, may indicate a repetitive data allocation that allows for thetransmission of data (step 66) to be periodically implemented.

Referring to FIG. 3B, a method 80 in accordance with the presentinvention for monitoring the quality of a communication channel betweenthe Node B 54 and a UE 82 is shown. The Node B 54 transmits data to theUE 82 (step 84). The UE 82 receives the data and transmits a signalquality indicator (step 86) such as the channel quality index (CQI) tothe Node B 54. This signal quality indicator may then be used as theselected quality indicator in step 60 of FIG. 3A.

It would be noted by those of skill in the art that steps 84 and 86 arenot necessarily sequential in practice. For example, in the FDD mode,signal quality indicators are periodically sent from the UE 82regardless of whether or not a data is transmitted. In such a case, theUE 82 transmits a signal quality indicator either periodically or inresponse to a specific event to the Node B 54. This signal qualityindicator may then be used as selected quality indicator in step 60 ofFIG. 3A.

As aforementioned, the selected quality indicator may be internallygenerated by the Node B, or externally generated by another entity suchas the UE and sent to the Node B. In accordance with a first embodiment,the criterion is the channel quality feedback from the UE. In thisembodiment, the CQI which is an indicator of the downlink channelquality is used.

In a second embodiment, the criterion is the ACK or NACK that the UEproduces in accordance with the H-ARQ process. For example, the numberof ACKs and/or the number of NACKs over a certain time period can beused to derive an indication of the quality of the channel.

In a third embodiment, the criterion is the choice by the Node B of themodulation and coding set (MCS) that is needed to successfully transmitdata. As would be understood by those of skill in the art, a very robustMCS is used when channel conditions are poor. Alternatively, a lessrobust MCS may be utilized when the channel conditions are good and alarge amount of data may be transmitted. The choice of the most robustMCS set may be utilized as an indicator of poor channel qualityconditions, whereas the use of the least robust MCS may signify thatchannel quality conditions are favorable.

In a fourth embodiment, the criterion is the depth of the queue insidethe Node B transmission buffer(s). For example, if the Node B 54 bufferis currently storing a large amount of data, it is an indicator thatchannel quality conditions may be poor since the data is “backing up” inthe Node B buffer. A buffer which is lightly loaded may be an indicatorthat channel quality conditions are good and the data is not backing up.

In a fifth embodiment, the criterion is the amount of data that is“dropped” in the Node B. As understood by those of skill in the art, thedropped data is data which the Node B has attempted to retransmitseveral times and has given up after a predetermined number of retries.If a large number of transmissions are dropped by the Node B, it is anindicator that channel quality conditions are poor.

In a sixth embodiment, the criterion is the amount of data that can betransmitted by the Node B within a predetermined duration, such as onehundred milliseconds. Depending upon the quality of a communicationchannel, the number of PDUs that are buffered in the Node B may change.Although the predetermined duration may be fixed, due to changingchannel quality conditions the amount of PDUs that may be transmittedwithin the predetermined duration may change dramatically. For example,if channel quality conditions are good, a hundred PDUs may be able to betransmitted within a hundred millisecond duration; whereas if channelquality conditions are very poor, only ten PDUs may be able to betransmitted within the hundred second duration.

It should be understood by those of skill in the art that other criteriawhich may directly or indirectly indicate the condition of the channelmay be utilized in accordance with the present invention. Additionally,a combination of two or more of the above-described criteria may beutilized or weighted accordingly, depending upon the specific needs ofthe system users.

Referring to FIG. 4, the benefits of adaptively controlling the flow ofdata between the RNC and the Node B can be seen. This example is thescenario where a retransmission is required due to a failed transmissionand the flow of data between the RNC and the Node B is decreased. As aresult of the data flow decrease, only one additional PDU with SN=8 isqueued in front of the retransmitted PDU with SN=3. The exertion of flowcontrol as shown in FIG. 4 reduces the latency of the retransmission ofthe PDU with SN=3 as compared to the prior art handling ofretransmissions as shown in FIG. 2 where the PDUs with SNs=8 and arequeued in front of the PDU with SN=3. Therefore, the PDU with SN=3 canbe retransmitted to the UE earlier. The in-sequence delivery requirementresults in faster processing and delivery of PDUs 4 through 8 to higherlayers.

While the present invention has been described in terms of the preferredembodiment, other variations which are within the scope of the inventionas outlined in the claims below will be apparent to those skilled in theart.

What is claimed is:
 1. A method for use in a Node B having a buffer, themethod comprising: monitoring an amount of data within the buffer;monitoring an amount of additional data the buffer may accept;calculating a capacity allocation; transmitting the capacity allocation;and receiving data in response to the capacity allocation, wherein thereceived data is equal to or less than the capacity allocation.
 2. Themethod of claim 1 wherein the buffer comprises a plurality of buffers orsub-buffers.
 3. The method of claim 1 further comprising; receiving achannel quality index (CQI); and calculating the capacity allocationbased on the CQI.
 4. The method of claim 3 wherein the CQI istransmitted by a wireless transmit/receive unit (WTRU) to the Node B inresponse to receiving data from the Node B.
 5. The method of claim 1further comprising; receiving a capacity request from a radio networkcontroller (RNC), the capacity request indicating an amount of data thatthe RNC has available to send to the Node B, wherein the Node Btransmits the capacity allocation to the RNC in response to receivingthe capacity request.
 6. A Node B comprising: a buffer; a processorconfigured to monitor an amount of data within the buffer, monitor anamount of additional data the buffer may accept, and calculate acapacity allocation; a transmitter configured to transmit the capacityallocation; and a receiver configured to receive data in response to thecapacity allocation, wherein the received data is equal to or less thanthe capacity allocation.
 7. The Node B of claim 6 wherein the buffercomprises a plurality of buffers or sub-buffers.
 8. The Node B of claim6 wherein the receiver is further configured to receive a channelquality index (CQI), and the processor is further configured tocalculate the capacity allocation based on the CQI.
 9. The Node B ofclaim 8 wherein the CQI is transmitted by a wireless transmit/receiveunit (WTRU) to the Node B in response to receiving data from the Node B.10. The Node B of claim 6 wherein the receiver is further configured toreceive a capacity request from a radio network controller (RNC), thecapacity request indicating an amount of data that the RNC has availableto send to the Node B, wherein the transmitter is further configured totransmit the capacity allocation to the RNC in response to receiving thecapacity request.